Session K14: High Energy Cosmic Rays

First results from the EUSO-Balloon campaign

EUSO-Balloon is a prototype detector of the Extreme Universe Space Observatory on the Japanese Experiment Module (JEM-EUSO). JEM-EUSO is a planned cosmic ray detector for the International Space Station (ISS). EUSO-Balloon was flown successfully as a balloon payload from the Timmins Stratospheric Balloon Launch Facility in Ontario, Canada the night of August 24/25. The time at float altitude was 4 hours. Three light sources, including a UV laser, were flown in a helicopter under the balloon, for 2 hours, to mimic the optical signatures of extensive air showers We describe Timmins campaign and present first results.   

UHECR mass composition measurement at Telescope Array via stereoscopic observation

The masses of primary ultra-high-energy cosmic-ray (UHECR) nuclei cannot be measured directly on an individual basis, but constraints on the chemical composition can be inferred from the distributions of observable properties. The atmospheric slant depth at which a UHECR-induced extensive air shower reaches its maximum number of particles, $X_{\rm max}$, is particularly sensitive to the mass of the incident nucleus, occurring earlier in the shower's longitudinal development for heavier nuclei at a given energy. The Telescope Array in west-central Utah, the northern hemisphere's largest UHECR detector, is equipped for accurate $X_{\rm max}$ and energy measurements via stereoscopic fluorescence observation. Using data from seven years of operation, we will present $X_{\rm max}$ distributions at several energies $E > 10^{18.2}~{\rm eV}$, and compare them to distributions predicted by detailed detector simulations under an assortment of assumed UHECR compositions and high-energy hadronic interaction models.   

Results on composition from the Pierre Auger Observatory

Cosmic rays have now been observed for over a century. The nature and origins of the ultra high energy cosmic rays are questions that have yet to be definitively answered. The southern Pierre Auger Observatory, located in Argentina, is currently the world's largest detector of ultra high energy cosmic rays. With unprecedented amount of data collected, it is shedding light to questions on the what these energetic particles are by its spectrum and composition determination. We report on the recent results on composition from the Auger experiment.   

Subluminal Events in the Search for UHECR at TA

The Telescope Array (TA) experiment in western Utah searches for ultra-high-energy cosmic rays (UHECR), which are particles of extraterrestrial origin with energies of 10$^{18}$ eV or more. These particles are ultra-relativistic nuclei that, on interaction with Earth's atmosphere, create showers of relativistic daughter particles. These showers should progress through the atmosphere with speeds near c, and observations so far confirm this expectation. However, velocity fits to detector data can return subluminal cases, which are expected to result from finite detector resolution and event misreconstructions. We have developed an event quality measure that can estimate the statistical significance of these slower-than-c cases. Our results indicate a low probability of real subluminal events in fluorescence detector data from TA.   

Fluoresence Detection of Cosmic Ray Air Showers Between $10^{16.5}$ eV and $10^{18.5}$ eV with the Telescope Array Low Energy Extension (TALE)

The Telescope Array Experiment has been observing cosmic ray air showers at energies above $10^{18}$ eV since 2008. TA operates three Fluorescence Detector (FD) sites, with telescopes that observe 3-31 deg in elevation. The FD sites are located at the periphery of a surface array of 507 scintillation counters covering 700 $km^{2}$, with 1.2km spacing. The TA Collaboration has completed building a low-energy extension at its Middle drum FD site. Ten new telescopes currently observe between 33 and 59 degrees in elevation. A graded ground array of between 400 and 600m will be placed in front of the TALE FD. With these upgrades, the physics threshold of TA will be lowered to $10^{16.5}$ eV. The TA Low Energy Extension(TALE) will explore the energy regime corresponding to that of the LHC in center-of-mass frame. This is also the range where the transition from galactic to extra- galactic cosmic ray flux is suspected to occur. We will give a brief overview of the physics, and report on the progress of TALE toward measuring the cosmic ray spectrum between $10^{16.5}$ eV and $10^{18.5}$ eV.   

Extending the measurement of shower maximum to the highest energies using universality and data from the surface detector of the Pierre Auger Observatory

The determination of the mass composition of ultra high energy cosmic rays is key to understanding their origin. The depth in the atmosphere at which the number of secondary particles in an air shower reaches a maximum, $X_{\rm max}$, is the best proxy for the mass of the primary particle that initiated the air shower. Existing measurements of $X_{\rm max}$ are based on the longitudinal development of air showers observed with fluorescence detectors. Thus, they lack statistics for energies above $\sim10^{19.4}$ eV due to the duty cycle of the telescopes and the stringent event selection. On the other hand, the value of $X_{\rm max}$ can be inferred for cosmic rays observed with surface detectors using the fundamental principle of shower universality. We will present our preliminary results for the measurement of $X_{\rm max}$ (mean value and standard deviation) as a function of energy extended up to $10^{20.1}$ eV using ten years of cosmic ray data recorded with the surface detector of the Pierre Auger Observatory.   

Uncertainties in energy reconstruction of cosmic rays for ANITA III caused by differences in models of radio emission in atmospheric showers

The third flight of the high-altitude balloon-borne Antarctic Impulsive Transient Antenna (ANITA III) was launched on a high-altitude balloon from McMurdo, Antarctica on December 17th, 2014 and flew for 22 days. It was optimized for the measurement of impulsive radio signals from the charged component of extensive air showers initiated by ultra-high energy cosmic rays in the frequency range $\sim 180 - 1200$ MHz. In addition it is designed to detect radio impulses initiated by high-energy neutrinos interacting in the Antarctic ice, which was the primary objective of the first two ANITA flights. Based on an extensive set of Monte Carlo simulations of radio emissions from cosmic rays (CR) with the ZHAireS and CoREAS simulation packages, we estimate uncertainties in the electric fields at the payload due to different models used in the two packages. The uncertainties in the emission are then propagated through an algorithm for energy reconstruction of individual CR showers to assess uncertainties in the energy reconstruction. We also discuss optimization of this algorithm.   

Overview and initial results from the ANITA HiCal Experiment

The ANtarctic Impulsive Transient Antenna (ANITA) is a balloon-borne apparatus that surveys the Antarctic ice looking for radio signals produced by ultra-high energy (UHE) neutrinos and cosmic rays. Neutrino signals originating from shower events in the ice and cosmic ray signals originating from shower events in the atmosphere arrive at ANITA after being, respectively, transmitted through or reflected from the ice surface. Since these signals interact with the air-ice interface, it is important to understand the impact of the transmission or reflection on the signal, specifically decoherence caused by surface roughness, in reconstructing the properties of the initial UHE particle. HiCal is a calibration pulsing unit employing a piezo-electric sparking device coupled to a dipole antenna that transmits a UHE-like impulsive signal. The first HiCal payload was launched on a second balloon in conjunction with ANITA-III, with the objective of transmitting pulses that would be received by ANITA both directly and as signals reflected from the ice surface. A ratio of the amplitudes of reflected to direct signals would provide a direct measurement of any decoherence effects caused by surface roughness. The design, testing, and initial results from the first HiCal flight will be discussed.   

Upgrade of the Central Laser Facility at the Pierre Auger Observatory and first results

The Pierre Auger Observatory (PAO) explores the nature and origin of cosmic rays with energies above 10$^{18}$ eV. It uses a hybrid technique that combines a Fluorescence Detector (FD) and a 3000 K$m^2$ surface Detector (SD) array. Two laser test beam facilities are located near the center of the observatory. The Central Laser Facility (CLF) and the eXtreme Laser Facility (XLF) track the atmospheric conditions during FD's operations and perform additional functions. The CLF was upgraded substantially in 2013 with a solid state laser, new generation GPS, robotic beam calibration system, and better thermal and dust isolation. The upgrade also includes a back scatter Raman Lidar receiver, providing an independent measurement the aerosol optical depth (tau(z,t)). We describe the new features, capabilities, and applications of the updated instrument, including, tau(z,t) calculations for atmospheric monitoring using a data normalized method, laser energy calibration, and steered laser firing for arrival directions studies. We also present the first tau(z,t) results after the upgrade,using two using two independent techniques. One method uses the FD's measurements of the CLF's laser shots in bi-static configuration. The other uses the Raman LIDAR in back scattered configuration.